·
Some devices may not able to look at the IPP or
DSCP fields
o
For instance, an MPLS LSR make QoS decision on
based on the 3-bit MPLS EXP field in the MPLS label
§
But unable to look at the encapsulated IP header
and DSCP field
o
In such cases, QoS tools configured on edge
devices to look at the DSCP & then mark a different field.
·
The non-IP header markable fields exist in only
parts of the network.
o
Those fields can be used for classification or
marking only on the appropriate interfaces
·
The rules for where these fields (CoS, DE, CLP,
EXP) can be used are as follows:
o
For
Classification:
§
On ingress only, and only if the interface
supports that particular header field
o
For
Marking:
§
On egress only, and only if the interface
supports that particular header field
Summarizes the QoS
marking fields
|
Field
|
Location
|
Length
|
|
IP Precedence
|
IP header
|
3bits
|
|
IP DSCP
|
IP header
|
6 bits
|
|
DS field
|
IP header
|
1byte
|
|
ToS byte
|
IP header
|
1byte
|
|
CoS
|
ISL & 802.1Q header
|
3bits
|
|
Discard Eligible(DE)
|
FrameRelay header
|
1bit
|
|
Cell Loss Priority(CLP)
|
ATM cell header
|
1bit
|
|
MPLS Experimental
|
MPLS header
|
3bits
|
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